This invention relates in general to color television picture tubes, and in particular to an improved method for establishing certain components within the picture tube envelope at a common electrical potential.
The color television cathode ray picture tube conventionally comprises a glass bulb including a funnel and a faceplate sealed to the flared end of the funnel. An electron gun is mounted in the funnel neck to provide one or more electron beams. The faceplate of the tube has a concave inner surface upon which is deposited groups of phosphors excited by the electron beam or beams. A shadow mask for color selection is attached in precise relationship to the faceplate by means of a plurality of suspension members which detachably engage metal studs extending from the faceplate. An electrically conductive coating, usually a composition comprising colloidal graphite, is applied to the internal surface of the funnel, and a high voltage is in turn applied to this coating from an external power source through an "anode button" in the funnel wall. The high voltage is usually termed the ultor anode potential.
It is common practice to "aluminize" the electron-excitable phosphor screen, a process which comprises the application of an electron-pervious film of aluminum over the phosphor deposits. The film increases the brightness of the display by acting as a mirror to reflect toward the viewer the light produced by the phosphors when excited. The thickness of the aluminum film is typically about 2000 angstroms. For maximum brightness of the display and for brightness uniformity, it is necessary that the aluminum be as smooth and mirror-like as possible, and devoid of blemishes such as holes or blisters. Other necessary qualities include firm adherence of the film to the phosphor layer, and uniform thickness of the film for uniform electron penetration. A problem arises in achieving these qualities primarily because of the unsmooth characteristic of the phosphor deposits. The problem is largely resolved by depositing a film of an organic material such as a lacquer on the phosphor deposits before application of the aluminum. The film acts to fill in uneven areas of the phosphor deposits, and provides a surface upon which the aluminum film can be deposited and take on the relatively smooth characteristic of the lacquer.
It is essential for proper cathode ray tube operation that the high voltage on the conductive coating of the funnel be also present on both the aluminum film and the shadow mask. The electrical path for establishing these components at a common ultor anode potential commonly comprises a spring extending from the shadow mask which is in contact with the high-voltage-charged funnel coating. The potential on the shadow mask is then conducted to the aluminum film through the shadow mask suspension springs which are attached to the metal studs which extend from the faceplate. The studs, which are embedded in the faceplate, are intended to be an electrical contact with the aluminum film; however, this contact often fails to be made. To ensure that the studs are in positive electrical contact with the aluminum film, it has been common practice to paint an electrically conductive "mustache" between the studs and the aluminum coating. The conductive material in common use has comprised a water-soluble silicate in a form suitable for application by a brush. The water is driven off in a subsequent tube baking process, leaving a hard, electrically conductive film between the studs and the aluminum film.
The use of such water-based solutions, which commonly contain either potassium silicate or sodium silicate as a binder, has resulted in a recurrent problem in that the water soluble solution did not "wet" properly, nor did it adhere to components. As a result, fragments of the coating often flaked off the studs and glass areas and become migrant particles. Such migrant particles can occlude one or more apertures in the nearby shadow mask; it is to be noted that the occlusion of even one such aperture is visible to the viewer. Also, the particles can migrate to the gun area and be the cause of inter-electrode arcing or cathode poisoning.
It is known in the art to use a conductive material comprising a mixture of silver with an organic solvent. Any benefits of the material are offset by problems such as its costliness and difficulty of application. Also, silver-bearing compounds usable in this application tend to become more electrically resistive with time, with the resistance increasing to a point where the performance of the coating as an effective conductor is marginal.
To make the water-soluble coating adherent, it has been necessary to introduce an extra baking cycle into the production process. After aluminizing and before applying the conductive mustache, the tube is baked at a temperature of about 400 degrees centigrade. Baking at this temperature eliminates the organic film by oxidizing it. However, as the film can as well be removed by the subsequent frit cycle bake, the requirement for a separate bake merely to promote adherence of the conductive mustache is a costly and energy-wasting step.